1 Technical Field of the Invention The present invention relates to an apparatus for activating occupant restraint devices of a vehicle.
2 Description of the Related Art
A conventional apparatus for activating occupant restraint devices of a vehicle has first and second acceleration sensors to detect deceleration of the vehicle.
Two output signals, which are generated by each of the first and second acceleration sensors of the vehicle, are fed to the same CPU of the vehicle. Then, the CPU determines whether or not to activate the occupant restraint devices of the vehicle such as an airbag, through two separate ways where each of the output signals of the first and second acceleration sensors of the vehicle are processed. As a result, the occupant restraint devices of the vehicle are activated upon receiving two activation signals from each of the two ways of determination. However, since both of the two output signals are processed by the single CPU, there is a possibility of faulty activation of the occupant restraint devices of the vehicle due to failure of the CPU.
To avoid faulty activations of the occupant restraint devices of the vehicle as described above, it can be considered to allow the apparatus to have two CPUs running in parallel. Thus, one CPU determines whether or not to activate the occupant restraint devices of the vehicle depending solely on the output signal of the first acceleration sensor of the vehicle, while the other executes the identical determination process depending solely on that of the second acceleration sensor of the vehicle.
However, the use of more than one CPU makes the apparatus expensive. As an alternative cost-effective solution, as proposed by Japanese Patent Application Laid-Open No. Hei 9-222437, the determination depending solely on the output signal of the first acceleration sensor is executed by using a CPU, whereas the determination depending solely on the output signal of the second acceleration sensor of the vehicle is implemented by using a relatively low-cost comparator instead of a CPU. The comparator is so configured to provide a signal to activate the occupant restraint devices of the vehicle when the output signal of the second acceleration sensor indicative of deceleration of the vehicle exceeds a predetermined threshold.
On the other hand, a RTTF (i.e., Required Time To Fire) for a vehicle, which is a required time-interval between an occurrence of collision of the vehicle and the resultant deployment of occupant restraint devices of the vehicle, is desired to be short.
However, when determining whether or not to activate the occupant restraint devices of the vehicle depending on the output signal of the second acceleration sensor of the vehicle by means of a comparator, an activation signal from the comparator cannot be outputted by a predetermined RTTF for the vehicle if there is a deviation of zero-point value for the second acceleration sensor of the vehicle. The deviation of zero-point value of the second acceleration sensor of the vehicle may be caused by, for example, aged deterioration of the sensor or a change in environmental temperature.
For example, there is an output signal of the second acceleration sensor of the vehicle as shown in FIG. 3, which is generated immediately after a collision of the vehicle. The output signal of the second acceleration sensor indicates a change in deceleration (as well as acceleration) of the vehicle with respect to time. In addition, the RTTF for the vehicle is predetermined as 26 msec.
First, suppose that the zero-point value of the second acceleration sensor coincides with a reference zero-point value thereof, and a threshold of deceleration for the comparator is predetermined as 70 m/s2. Then, in FIG. 3, when the output signal of the second acceleration sensor goes above the solid horizontal threshold line which corresponds to the threshold of 70 m/s2, the comparator outputs a signal to activate the occupant restraint devices of the vehicle.
Now, suppose that the zero-point value of the second acceleration sensor deviates, for example a value of 50 m/s2, towards the negative direction of deceleration (i.e., the positive direction of acceleration) in FIG. 3. Then, the effect of the deviation is equivalent to that of a change where the output signal of the second acceleration sensor remains in the initial position, while the threshold for the comparator increases from 70 m/s2 to 120 m/s2. Referring to FIG. 3, there are a dashed horizontal line corresponding to the increased threshold of 120 m/s2, and a solid vertical line representing the RTTF of 26 msec for the vehicle. It will be noted from FIG. 3 that the comparator cannot output a signal to activate the occupant restraint devices of the vehicle by the RTTF of 26 msec due to the deviation of zero-point value of the second acceleration sensor.